Method and apparatus for determining the preload for screws for dental implant systems

ABSTRACT

A method and apparatus are provided for determining the preload in a dental implant system. The preload is determined by transmitting a sonic impulse, which is preferably an ultrasonic impulse, at a predetermined frequency to the head of the implant screw through a transducer, which may be incorporated into the head of the screw, the head of a wand which generates the sonic impulse, or the transducer and pulse-generating instrumentation may be incorporated into a torque generating instrument used to tighten the screw. The preload is determined by measuring the delay between the first and second reflections through the preloaded screw to determine a preload value and comparing that value with a pre-established baseline value for the screw, and comparing the difference with a predetermined table of values to determine the preload on the screw.

[0001] This invention was made with government support under grantnumber 1R43DE13454 awarded by National Institutes of Health Office ofExtramural Programs SBIR/STTR Grant Programs. The government has certainrights in the invention.

BACKGROUND

[0002] This invention relates to a method and apparatus for ensuringthat small screws used to hold together dental implant components aretightened to the correct initial stress level, or “preload.” Accordingto the National Institute of Health, among the factors involved in thedesign of a dental implant are the forces produced during implantloading, the dynamic nature of loading, and the mechanical and structureproperties of the prosthesis in stress transfer to tissues.Unfortunately, accurate data on such parameters are incomplete. NationalInstitutes of Health Consensus Development Conference Statement onDental Implants. June 13-IS, 1988.

[0003] During the early 1970's the dental profession was very hesitantto use dental implants or fixtures surgically implanted into a patient'sjawbone as a treatment option to replace missing teeth. However, successwith implants in the past 30 years has replaced this skepticism. This isdue to the efforts of P-I Brånemark and co-workers in Sweden whointroduced the concept of osseointegration in humans. When theprinciples of osseointegration are followed, the anchorage of anon-biological titanium implant unit to living bone will occur, withapproximately 95% and 85% implant survival rates for the lower and upperjaws, respectively. See, for example in U.S. Pat. Nos. 4,824,372,4,872,839 and 4,934,935 to Jorneus et al., Brajnovic and Edwards,respectively.

[0004] One of most critical aspects in the replacement of missing teethusing dental implants is the ability of small screws positioned withinthe implant complex to hold the various implant parts together duringloading and stress transfer. As any screw in the implant system istightened, the initial stress level developed within the screw becomescritical to the maintenance of the joint stability between the parts thescrew is clamping together. Owing to the high strain level that theassembled joint experiences. in everyday life, this initial stress levelcalled the preload is of paramount importance. Insufficient tighteningof a screw in the implant system can result in the screw becoming looserather quickly, and over time this looseness can lead to fracture of thescrew and potentially failure of the implant reconstruction. This isparticularly critical for screws that secure spacers or abutments to theimplant or fixture.

[0005] The stability of the screw joint is considered a function of thepreload stress achieved in the screw when applying the preloadtightening torque to clamp the implant components together. The optimumpreload torque is influenced by the geometry of the screw, the contactrelationships between the screw and its bore, between the screw and itsthreads, and between the bearing surfaces of the components clampedtogether by the screw, friction, and the properties of the materialsused. One example is the joint formed between the bearing surface of theimplant and the bearing surface of the spacer or abutment. Anotherexample is the joint formed between a prosthesis and an abutment, alsoheld together by a small screw in the implant system.

[0006] When the screw joint experiences instability, the screw willeither loosen or fracture. Screw joint failure occurs in two stages. Thefirst stage consists of external functional loading applied to the screwjoint that gradually leads to the effective erosion of the preload inthe screw joint. Any transverse or axial external force that causes asmall amount of slippage between the threads releases some of thestress, and therefore, some of the preload is lost. The greater thepreload applied to a screw joint (up to a maximum equal to theproportional limit), the greater the resistance to loosening and themore stable the joint. As long as the frictional forces between thethreads remain large, a greater external force will be required to causeloosening.

[0007] Once the critical load exceeds the screw joint preload, itbecomes unstable. The external load rapidly erodes the remaining preloadand results in vibration and micromovement that leads to the screwbacking out. Once this second stage has been reached, the screw jointceases to perform the function for which it was intended and has failed.

[0008] Optimizing the preload of a screw used in a dental implant systemis critical for implant screw joint stability. As was stated earlier,implant screw loosening and fractures are quite common. The fact that onaverage complications with implant screw will occur in one out of everyfour implants surgically placed is significant. The need for optimumpreload in screw tightening at the initial stages of implant componentassembly and completion of the final implant restoration cannot be leftto chance. An instrument that scientifically records the preloadestablished in these implant screws following tightening and prior toany external load applications is essential to implant performance andthe quality of life of the patient who receives implants as part oftheir dental rehabilitations.

[0009] It has been reported by Patterson and Johns that to achieve themaximum preload possible in component screws for dental implants, it isnecessary to apply the appropriate tightening torque to each screw.Torque tightening devices for implant screws are discussed, for example,in U.S. Pat. No. 6,109,150 and 5,626,474. However, most screwtorque-tightening devices lack accuracy because of a number of variablesbeyond the control of these conventional instruments. This means thatthe maximum stress developed in an implant screw tightened byconventional torque-tightening devices may be less than 70% of the yieldstrength of the screw itself and therefore well below the maximumpossible preload for a stable joint. If the screw is loaded to theappropriate preload level one can be confident that the screw will notfail during the life of a patient when “normal” external loads areapplied.

[0010] Ultrasound instrumentation has been used to measure the preloadestablished in large bolts and screws in industrial applications. Thusfar, however, it has not been applied to small screws the size of thoseused in implant systems. In industrial applications for large bolts andscrews, the most common ultrasonic instruments for control of screwtension are called “pulse-echo” or “transit time” instruments. Bickfordhas described the use of this method with large bolts. A drop of fluidis placed on the head of the bolt to reduce the acoustic impedancebetween the transducer and bolt head. An acoustic transducer of somesort is placed against the bolt head. The instrument is then zeroed forthis particular bolt because each bolt will have a slightly differentacoustic length even if their physical lengths are the same. The zeroload is recorded before tightening. Next, the bolt is tightened. If thetransducer can remain in place during tightening, it will show thebuildup of stretch or tension in the bolt during tightening. If it mustbe removed, it is repositioned on the bolt again after tightening toshow the stress level achieved. If at some future time one wishes tomeasure the tension present within the bolt, the original data can beinput to the instrument computer unit and after placing the transduceron the top of the bolt, the instrument will record the existing tensionand the zero stress conditions.

[0011] In principle, the electronic instrument delivers a voltage pulseto the transducer, which emits a brief burst of ultrasound (typicallyfive to seven or more cycles). This burst passes down through the bolt,echoes off the far end, and returns to the transducer. The electronicinstrument measures very precisely the time delay required for the burstof sound to make its round trip in the bolt. As the bolt is tightened,the amount of time required for the ultrasound to make its round tripincreased for two reasons: 1) the bolt stretches as it is tightened, sothe path length increases, and 2) the average velocity of sound withinthe bolt decreases because the average stress level has increased. Atlow strain those functions can be approximated by linear ones of thepreload in the bolt, so the total change in transit time is also alinear function of preload.

[0012] In dental implant technology, it is important to know whatpreload exists in implant screw joints at any time during implanttherapy and throughout the life of the implant.

[0013] All of the currently used implant screws are fabricated frommaterials that are nontransparent and nonmagnetic. No other efficienttechnique for stress measurements of nonmagnetic and nontransparentmaterials is available. In contrast, a magnetic hysteresis curve can beused to infer the stress in magnetic materials, and also opticalcoherent methods can be used to infer the stress in transparentmaterials. However, the accuracy of this latter method is significantlylower than that of the ultrasonic TOF measurements, and as stated theimplant screws are made of nonmagnetic materials. The use of mechanicalmethods for stress measurements requires exact measurements of thelength of the implant parts, and with the 30 plus implant manufacturersthroughout the world and their reluctance to provide this data, thismethod has definite limitations.

[0014] Ultrasonic measurement of the stress in a screw or bolt with arelatively big cross-section and length has been known for some time.Since the early fifties the technique has been theoretically andexperimentally proven for a range of materials. Experimental andtheoretical results obtained by Huges and Kelly on samples of rail steelwith various load conditions have shown the proportionality between theuniaxial stress and velocity of acoustic waves. However, since then themethod has been used for only relatively long and large cross-sectioncomponents, partially due to an insufficient accuracy of TOF measuringdevices. At present a digital oscilloscope's sampling rate ranging toseveral gigahertz makes possible a real time measurement of timeintervals with the 10-100 picosecond accuracy. As to the dental implantscrew in question, the ultrasonic evaluation of the stress via the timeof flight measurement in principle is feasible. In practice the methodis not straightforward and several factors have the potential toinfluence the accuracy, however, the whole performance is predictable.Difficulties reside in the small size transducer required (around 0.5mm. active element diameter), and the small length inducing a lowvariation of the time of flight of the ultrasonic pulse. The smaller thetransducer, the greater the exposure to a stronger mechanical stress.The smaller the length of the screw, the less variation in the time offlight and consequently the lower precision of the stress measurementobtained. Ambient temperature influencing elastic properties ofmaterials, could also be a concern, which can be controlled.

[0015] The optimum preloads suggested for implant screw joints are apercentage of the yield strength of the screw. For example, 50-60% ofyield has been suggested for average nongasketed joints, with “normal”safety or performance concerns. A 70-75% of yield has been suggested asthe upper limit for nongasketed joints where “low preload” problems havebeen experienced in the past such as leaks, self-loosening, fatigue,etc. Joints which have had consistent “low preload” problems in thepast, and where the need to avoid failure is significant and whereservice loads (or ignorance of service loads) make it unwise to take thescrews any closer to the yield point, a 85-95% of yield has beensuggested. Obviously, the preloads suggested for various screw jointsdemonstrate considerable variation, and depending on the jointrequirements, the amount of preload achieved (% of the yield) would besignificant in the performance of the joint. Furthermore, the amount ofpreload suggested depends on the accuracy of knowing the yield point ofthe screw. McGlumphy has reported significant differences between screwsfrom several implant manufacturers even though the suggested tighteningforces, and thus the preload achieved for these screws were the same.The force needed to cause failure in abutment screws for the systems astested by McGlumphy ranged from 1.22 to 17.23 kg. However, even if theultimate tensile strength of the screw, the proportional limit and theelastic range were known, neither the preload created by tighteningusing a torquing device suggested by the manufacturer for the particularscrew nor the variability in the preload as a result of the tighteninginstrument used by the operator is known.

[0016] In summary, it would appear that a great deal of subjectivityexists in the tightening of implant screws. It isn't any wonder thatscrews loosen or fracture. The tightening instruments are a majorvariable. The quality and quantity of the tightening torque is inquestion. The “target” preload is uncertain. Finally, the achievedpreload is unknown. In implant joints, which are very critical jointassemblies, the stability of the joint begins with knowing the exactpreload achieved following the clamping together of the components. ThePreload Measurement Gage will provide clinicians with that information.

SUMMARY OF THE INVENTION

[0017] This invention provides a method of determining the preload on ascrew used in an implant system that secures a component to a fixture orto another component in a dental implant system which comprises thesteps of transmitting a sonic impulse at a predetermined frequency tothe head of the screw through a transducer when the screw is in anunstressed condition; measuring the delay between the first and secondreflections through the unstressed screw, and establishing a baselinevalue for the unstressed screw; applying a preload of a predeterminedvalue to the screw to secure the implant component in the implantsystem; transmitting a sonic impulse at a predetermined frequency to thehead of the screw through a transducer; measuring the delay between thefirst and second reflections through the preloaded screw to establish apreload value; and determining the difference in the delay between thebaseline value and the preload value, and comparing the difference witha predetermined table of values to determine the preload in the screw.

[0018] Transducers used in this invention may be any transducer thattransmits and receives sonic impulses. Preferably, the sonic impulse isan ultrasonic impulse. The frequency of the impulse may vary dependingon the material characteristics of the screw.

[0019] Screws used with this invention may be measured in this manner inthe unstressed state before they are packaged and sold, and the baselinevalue may be provided with the sales information.

[0020] This invention also includes apparatus for determining thepreload in a dental implant system that includes a fixture having oneend adapted for osseointegration into a jawbone, the other end adaptedto receive a spacer and including an internal bore that includes threadsfor engaging with a screw to secure the spacer to the implant, thespacer including an internal bore to receive the screw. The prosthesismay be attached to the implant system with a second screw. The apparatuscomprises means for achieving a preload in the screw to secure thecomponent in the implant system, which may be any conventional means,such as a hex wrench or screwdriver, as are commonly sold by companiessuch as Nobel Biocare, Implant Innovations, or others who market dentalimplants, abutments, and tools. The apparatus also includes means fortransmitting a sonic impulse, which is preferably an ultrasonic impulse,at a predetermined frequency to the head of the screw through atransducer, which may be any apparatus that generates an ultrasonicimpulse at the desired frequency. The frequency of the sonic impulse mayvary, depending on the material and configuration of the screw. Theapparatus also includes means for measuring the delay between the firstand second reflections through the preloaded screw to determine apreload value, which may consist of a suitable measurement circuit,which may be in a separate control box, or part of a wand used totransmit and receive the ultrasonic impulse and pulses. The apparatusalso includes means for determining the difference in the delay betweena pre-established baseline value for the screw and the preload value,and comparing the difference with a predetermined table of values todetermine the preload on the screw.

THE DRAWINGS

[0021]FIG. 1 is a sectional view of a dental implant installation of theprior art, adapted for purposes of this invention.

[0022]FIG. 1A is a sectional view taken along line A-A.

[0023]FIG. 2 is a block diagram of one embodiment of the apparatus ofthis invention.

[0024]FIG. 3 is a typical A-scan pattern of pulses generated by thisinvention.

[0025]FIG. 4 is a diagram that demonstrates the principal of the time offlight.

[0026]FIG. 5 is a depiction of a preload strain gauge program window fora computer program that may be used in the present invention.

[0027]FIG. 6 is a diagram showing configuration windows that may be usedwith the present invention.

[0028]FIG. 7 shows a diagrammatic depiction of a wand that may be usedwith the present invention.

[0029]FIG. 8 is a diagrammatic depiction of a second type of wand thatmay be used with the present invention.

DETAILED DESCRIPTION

[0030] Implant systems, which are well known in the art, generallyconsist of an implant or fixture, which is surgically implanted into apatient's upper or lower jawbone. As shown in FIG. 1 and FIG. 1A, thefixture 10 includes an externally threaded body 12, which is surgicallyscrewed into the jawbone. At one end of the body is a flange 14, whichhas bearing surface 16. Body 12 of fixture 10 includes an internal bore18, which extends from the flange 14 and which is at least partiallythreaded to receive an abutment screw (also known as a spacer screw) 22,which includes a threaded portion 24, and a head 26. An abutment 30includes a bearing surface 32, which forms a joint 34 with the bearingsurface 16 of flange 14. Abutment 30 also includes an internal bore 36to receive screw 22 and a flange 38 which is smaller in diameter thanthe head 26 of abutment screw 22. The abutment screw 22 passes throughthe bore 36 of abutment 30, and the threaded portion 24 of abutmentscrew 22 mates with the internal threads 20 of internal bore 18 of thefixture 10. Abutment screw 22 is screwed into the internally threadedbore 18 of fixture 10, and tightened to a predetermined pre-load tosecure the abutment 30 to the fixture 10.

[0031] Head 26 of the abutment screw 22 is provided with an internalbore 40 which has a geometric shape, such as an internal hex, adapted toreceive a tool such as a hex wrench for tightening the screw. Othergeometric shapes for tools are well known in the art. The abutment screwused for practicing the invention is provided with a reflecting surfaceat the bottom 42 of bore 40. A second reflecting surface 44 is providedat the opposite end of the screw. Each reflecting surface is preferablygenerally flat, and generally perpendicular to the line of transmissionof the sonic pulse. Any number of screw head designs may be used, solong as each end of the screw (heads and ends) has a reflecting surfacethat is sufficiently perpendicular to the ultrasound propagation pathwayin order to register and record at a sufficient amplitude the time offlight between two acoustical impulses traveling the length of thescrew. All dental implant screw designs can potentially be modified tocreate a sufficiently reflecting area within and at the base of the headalteration and also at the end of the screw for this purpose.Alternatively, other forms of reflecting surfaces may be used.

[0032] In one embodiment of the invention depicted in FIG. 1, a small 20MHz PZT element (transducer) 50 of 0.8 mm diameter is fixed to aflattened area 42 in the head 26 of a screw 22. This transducer 50provides the interface between the screw 22 and an acoustic source 70(See FIG. 2) for the transmission of an acoustic pulse along the longaxis of the screw.

[0033] As shown in FIG. 7 and FIG. 2., the acoustic source is a handheld wand 70 that is electronically connected to a control box 72. Theelectronic connection may be hard-wired 74, or it may be accomplishedremotely, such as by infrared or by so-called “bluetooth” technology, solong as the wand is provided with appropriate infrared transmissionand/or receiving means. Alternatively, the control box can be providedin miniaturized form through microelectronics entirely within the handle76 of the wand 70.

[0034] Within the control box 72 are the electronics needed to initiatean ultrasonic impulse from an acoustic source 78 near the small tip 82of the wand 70. The tip 82 of the wand 80 is placed in contact with thetransducer element 50 in the head 26 of the screw 22, which clampstogether the abutment 30 and implant 10 to form the screw joint 38 ofthe implant assembly. A sound impulse is initiated from the tip 82 ofthe wand 80 and the sound is transmitted by the transducer 50 in thescrew head 26 to the opposite end 44 of the screw 22. Two clearsequential echo-pulses are reflected from the screw bottom (end) back tothe transducer and ultimately across the interface to the wand.

[0035] The time of flight between pulses 1 and 2 can be determinedindependent of the acoustic contact variations. The time of flight ofthe wave propagation through the screw is registered by the transducer50 and the information is transmitted and processed in the control box72 by a computer microchip. Tightening of the screw will producedvariations in screw length related to the elastic properties of thescrew. Screw length variations influence the time of flight of theultrasonic pulse along the long axis of the screw. The differences inthe time of flight recorded before and after screw tightening are usedto compute the stress within the screw as a function of screwtightening. The stress is computed by the control box electronics, anddisplayed both graphically and digitally at the control box 72.

[0036] As shown schematically in FIG. 2, a system for preloadmeasurement may include, for example, an embedded 20 MHz ultrasonictransducer 50, an ultrasonic pulser-receiver USD-15 (Krautkramer) 72, adigitizing oscilloscope TDS-520 (Tektronix) 73 connected to a GPIB port(IEEE488) 75 with computer 77. As is discussed above, the implant screwis provided with a generally flattened surface inside the tool-receivingbore in order to accommodate a 1 mm diameter piezoelectric piston. Apiezoelectric disk 50 is positioned inside the screw head and two wiressoldered in order to provide the electric path. To protect thepiezoelectric element and the wiring the head was molded with epoxycompound. The setup immediately provided two clear echo-reflections fromthe opposite end of the screw. To increase amplitude of the reflectedsignals the threaded end of the screw was slightly flattened. In FIG. 3a typical A-scan is given. The basics of the measurement consist ofdetermining variation of the delay between 1^(st) and 2^(nd)reflections. To measure the TOF between the two pulses zerocross-section method is used. The software seeks for the first minimumof both signals and then calculates the time coordinate of next zerocross-section linearly interpolating the signal between two consecutivesamples for both first and second echo-pulses and finally estimates TOFas given by the following formula. $\begin{matrix}{{{TOF} = {\frac{1}{f_{sampl}}( {i - j + ( {\frac{{wfm}(i)}{{{wfm}(i)} - {{wfm}( {i + 1} )}} - \frac{{wfm}(j)}{{{wfm}(j)} - {{wfm}( {j + 1} )}}} )} )}},} & (1)\end{matrix}$

[0037] where f_(sampl) is the sampling frequency, wfm(k) is thedigitized waveform data, i, j are samples around zero crossing (see FIG.4). Better results are obtained at 1 GHz sampling rate. A real-timemeasurement provides ±0.2 ns precision, with 32-average mode theprecision goes to 0.02 ns. This corresponds approximately to 0.025° C.temperature variation, or 0.6N force variation using for approximationelastic parameters of mild steel. Exact values of these uncertaintiesare to be calculated after the stress-TOF and temperature-TOFcharacteristics are studied for the material used in manufacturing thescrew. This system results in excellent resolution of the method.

[0038] To realize the measurement method a software program may be used.Basic features of the program are transfer of the digitized A-scan fromTDS520 to a personal computer, serial port communication, time delaycompensation and measurement, and data storage. The outlook of a programwindow is given in FIG. 5. Configuration windows for the preload gaugeare shown in FIG. 6.

[0039] In another embodiment, depicted in FIG. 8, wand 90 is designed totransmit and receive acoustic and time of flight data without the needfor contact between the wand tip and a transducer located in the head ofthe screw. In this embodiment, the transducer 50 is positioned withinthe tip 92 of the wand, near enough to the end to transmit and detectsonic impulses. The wand also incorporates technology for digital analogsignals to be transmitted and received in order to carryout thefunctions identified in the hard-wired control box. The informationreceived and transmitted by the wand may be displayed in a remotelylocated display mode.

[0040] In another embodiment, the ultrasonic transducer may be locatedwithin the tool used to tighten the screw. Thus, a screwdriver may beused to tighten the screw and either simultaneously or at the end of thetorquing procedure measure the stress within the screw. One end of thescrewdriver is formed in a well known latch-type design for attachmentto an electronic or manual tightening torque apparatus. At the other endof the screwdriver, the ultrasonic transducer is positioned within thescrewdriver end in a position permitting it to transmit and detect sonicimpulses. The transducer is electronically connected to the latch-typeend by internal circuitry. The transducer is electronically connected toeither the electronic or manual tightening torque handpiece by anelectronic interface within the handpiece head. The screwdriver ispositioned in the screw bore and brought into intimate contact with thescrew. Following initiation of the sound impulse, the sound travelsthrough the screw to the end of the screw. In the electronic tighteningtorque apparatus, the time of flight of the wave propagation through thescrew is registered by the transducer in the screw driver, and theinformation is transferred electronically back to the tightening torqueapparatus control boxes or an associated display unit. The elasticproperties of the screw, which have been altered by the torquing forceused to tighten the screw, are displayed both graphically and digitallyat the control box (6) as the preload.

[0041] In the case of the manual tightening torque apparatus, theelectronics for initiation of the wave impulse from the screwdriver, anddata retrieval and processing are located in a modified handle for thetightening torque apparatus. The registration, recording and computationof the time of flight are performed using micro-processing technologyand transferring the information from the electronic port in the manualtightening torque handle (2) as a digital analog signal to a remotedisplay unit.

We claim:
 1. A method of determining the pre-load on a screw thatsecures a component to other components in a dental implant systemcomprising: a. Transmitting a sonic impulse at a predetermined frequencyto the head of the screw through a transducer when the screw is in anunstressed condition; b. Measuring a first delay between first andsecond reflections through the unstressed screw, and establishing abaseline value for the unstressed screw; c. Applying a preload of apredetermined value to the screw to secure implant components together;d. Transmitting a sonic impulse at a predetermined frequency to the headof the preloaded screw through a transducer; e. Measuring a second delaybetween the first and second reflections through the preloaded screw toestablish a preload value; and f. Determining the difference between thefirst delay and the second delay to establish a difference between thebaseline value and the preload value, and comparing the difference witha predetermined table of values to determine the preload on the screw.2. The method of claim 1 wherein the sonic impulse is an ultrasonicimpulse.
 3. The method of claim 1 wherein the screw secures an abutmentto an implant.
 4. The method of claim 1 wherein the screw secures adental prosthesis to an abutment.
 5. The method of claim 1 wherein thescrew secures a dental prosthesis directly to an implant.
 6. Apparatusfor determining the preload on a screw in a dental implant system thatincludes at least one screw for securing components of the dentalimplant system together, wherein the dental implant system includes atleast a fixture having one end adapted for osseointegration into ajawbone, the other end adapted to receive an abutment, and the abutmentadapted to receive and support a dental prosthesis, the apparatusfurther comprising a. Means for applying a preload to the screw tosecure at least some components of the implant system together; b. Meansfor transmitting a sonic impulse at a predetermined frequency to thehead of the screw; c. Means for transmitting the sonic impulse throughthe screw and detecting first and second reflections of the pulses; d.Means for measuring the delay between the first and second reflectionsthrough the preloaded screw to determine a preload value; and e. Meansfor determining the difference in the delay between a pre-establishedbaseline value for the screw and the preload value, and comparing thedifference with a predetermined table of values to determine the preloadon the screw.
 7. The apparatus of claim 1 wherein the sonic impulse isan ultrasonic impulse.
 8. The apparatus of claim 1 wherein the screwsecures an abutment to an implant.
 9. The apparatus of claim 1 whereinthe screw secures a dental prosthesis to an abutment.
 10. The apparatusof claim 1 wherein the screw secures a dental prosthesis directly to animplant.
 11. The apparatus of claim 1 wherein the means for transmittinga sonic impulse is an apparatus that produces an ultrasonic impulse. 12.The apparatus of claim 1 wherein the means for transmitting the sonicimpulse through the screw and detecting first and second reflections ofthe pulses is a transducer.
 13. The apparatus of claim 12 wherein thetransducer is affixed to the head of the screw.
 14. The apparatus ofclaim 12 wherein the transducer is affixed to a wand, and the wand is inelectronic communication with an electronic control and measurementcircuit.